When seen in the cross-section, the ceramic decorated with inorganic pigments (inorganic coloring material) is routinely constructed by a substrate (body or ground) layer, a glazing layer and a coloring material layer. The coloring material layer, also termed a decorative part, is formed by coating a European style coloring material or a Japanese style coloring material over a glazing layer applied to a pale to white-colored substrate layer followed by firing, baking to be affixed in situ. The European style coloring material is a mixture of inorganic pigments manifesting a variety of color tones and a colorless transparent glass as a binder, whilst a Japanese style coloring material is comprised of colored glass powders obtained on smelting pigments in a colorless transparent glass.
Based on the relative positions of the glazing layer and the coloring material layer, the coloring decorative method for decorative porcelain may be roughly classified into an under-glazing coloring method, applying a substrate layer, a coloring material layer and a glazing layer, in this order, an in-glaze coloring method, applying a substrate layer, a coloring material layer partially or wholly precipitated into a glazing layer and a glazing layer, in this order, and an on-glaze coloring method, applying a substrate layer, a glazing layer and a coloring material layer, again in this order. Each of these three methods has its peculiar characteristics. Of these, the in-glaze coloring method is compromised between the other two methods as to the firing temperature and the structure. Moreover, with the in-glaze coloring method, a larger number of inorganic pigments can be used than with the under-glazing coloring method so that the color or the pattern may be expressed more profusely. In addition, with the in-glaze coloring method, since the firing temperature is higher than with the on-glaze (over-glaze) coloring method, the inorganic coloring material may be precipitated into the glazing layer, so that the chemical durability is higher even if the ceramic article is washed with acid or with alkali. Moreover, there is little risk of the coloring material layer being peeled off or detached despite wear or friction during mechanical handling such as during rinsing of various sorts.
In these years, the in-glaze coloring method has come to be used more frequently than the other methods. In particular, a hard ceramic ware (china ware) obtained on prompt firing with the use of the so-called RHK (roller hearth kiln) is now in widespread use. In this case, the firing temperature, that is the temperature of affixing the decoration, is close to the glost firing or biscuit firing temperature of the substrate layer, whilst the time required for firing is drastically reduced as compared to that in case of the conventional tunnel kiln, such that the firing may be completed in 60 to 150 minutes. Since the coloring-firing temperature is higher, there is imposed limitation as to the sort of the usable inorganic coloring materials. At this temperature, the inorganic coloring material melts or ‘sinks’ into the glazing layer of the porcelain, resulting in improved stability of the coloring material layer. Meanwhile, this melting or sinking feature accounts for an alternative appellation ‘sink-in coloring’ for the in-gaze coloring. As typical of this in-glaze coloring material method, the JP Patent Kokai JP-A-58-25983 discloses an in-glaze decorating method exploiting a transcription sheet. The method disclosed features coating a coloring materialing layer on the glazing layer with a glass flux layer and firing the resulting mass.
Although the firing temperature in the above-described various coloring methods differ with the composition of the substrate of the ceramic articles used, it is approximately 1100° C., approximately 900° C. and approximately 750° C. for under-glaze coloring, in-glaze coloring and in on-glaze coloring, respectively, for the soft porcelain, such as bone china. On the other hand, with the hard china ware, such as hotel china, it may approximately be 1400° C., 1250° C. and 820° C., respectively. The firing temperature is similar for the hard pottery, classified into semi-vitrified china.